Q. Can digital audio cables cause ground loops?

By Various

In SOS December 2008, you explained how USB and Firewire cables could complete ground loops and result in audio problems. Is the same true of digital audio cables?

Philip Bramley

 
M-Audio's Audiophile 192 is an example of a soundcard with well‑designed S/PDIF sockets. Their metalwork is insulated from the PCI backplate, and the small black component immediately above them on the circuit board is the pulse transformer for the output, which prevents ground loops.

SOS columnist Martin Walker replies: In theory, most digital connection formats incorporate safeguards to prevent ground loops. Unfortunately, in practice such problems do occasionally occur, either due to incorrectly wired cables or to shortcuts taken by the manufacturer of the gear in question. The tiny ground‑loop signals are rarely sufficient to result in corruption of the digital data itself, since that either gets through or it doesn't. However, if a digital audio connection completes a ground loop, it may indeed result in characteristic hums, digital whistles and whines appearing in your analogue audio signals. Here's a quick run‑down of possible problems you might experience, plus some ways to avoid them. Let's look at the easiest options first.

ADAT and S/PDIF optical both use fibre‑optic cables, giving true galvanic isolation, just like a transformer, so they can never result in ground loops. MIDI inputs also incorporate opto‑isolators, so plugging in a MIDI cable should never result in a ground loop. A few MIDI In ports (mostly on budget PC joystick/MIDI adaptors) may omit this circuitry, to save a few pennies, but the most common cause of MIDI ground loops is using an incorrectly wired cable. Avoid five‑pin DIN hi‑fi cables and use proper MIDI cables, which use only pins four and five for data transfer, and connect the cable screen to pin two only at each end of the cable, but not to the metal shell of the plugs themselves. If a multimeter shows zero resistance between the metal shell of the plugs at each end of a MIDI cable, discard it.

AES/EBU normally uses balanced connections with transformer isolation (signal 'pulse' transformers are used, which are the tiny black boxes you can often see on soundcards near their digital I/O sockets). It's very rare to run into AES/EBU ground‑loop problems, since these sockets generally appear on professional gear where no corners are cut.

The most problematic digital connections tend to be S/PDIF coaxial (phono) ones, which, although unbalanced, should, in theory, have transformer‑coupled outputs just like AES/EBU ones, so that when you connect them to an S/PDIF input no ground loop is created. Unfortunately, this is an area where corners get cut on budget audio interfaces, and if your S/PDIF phono sockets are bolted direct to the metalwork, no transformer has been used.

It's not wise to attempt removing the screen connection at one end of an S/PDIF cable, since this can cause signal degradation and data errors. If you suspect a coaxial S/PDIF connection is causing your problem, switch to Toslink optical, if both pieces of gear offer this option. M‑Audio's CO2 (around $60) is also a handy gadget to have, as it converts between coaxial and optical S/PDIF, and also features transformer‑isolated phono sockets so you can use it in‑line to cure ground loops.

Finally, it pays to be extremely systematic when tracking down digital ground loops, as I found recently. My studio developed annoyingly intermittent digital noises that disappeared as soon as I disconnected an S/PDIF lead between my audio interface and D/A converter. However, this connection wasn't, in fact, to blame. I had two half‑rack‑width devices bolted into a rackmount tray, and had scrupulously made sure neither of them touched the metalwork of the tray or each other. Unfortunately, the bolt heads used to secure them to the tray had eventually worked their way through the paintwork on the top panel of the rackmount device beneath, which then completed the circle of this particular ground loop. On this occasion, some nylon bolts cured the problem!

Published February 2009

Korg Kross -- Split Functionality

Q. Which mixer should I choose for drum recordings?

By Various

I'm looking for something to record my latest project with. I currently own an Apple iMac running OS X Tiger, and the main program I use for recording is Apple's own Soundtrack. I'm thinking of upgrading at some point to Logic Pro 8, but for the moment I'm with Soundtrack. I have eight Audio Technica mics for the drum kit and I'm really looking to record through a mixer into Soundtrack (Logic in the future). However, I still want to be in control of individual mic levels after I've recorded within Soundtrack or Logic, even having those individual mics on different tracks.

I'm wondering if you could help me in choosing some kind of mixer with eight XLR inputs that will let me control individual tracks within the drum mix afterwards?

 
Mackie's 1642 VLZ3 mixer, with eight good-quality mic preamps, looks like a good bet if multitrack drum recording is part of your studio workload.

George Barnett

SOS Reviews Editor Matt Houghton replies: Dilemmas like this are rather difficult to advise upon without knowing anything about your budget, but I'll try...

Let's start with the cheapest and simplest option. If you plan to do all the mixing and processing 'in the box', you don't really need a mixer at all: you just need an audio interface and sufficient mic preamps. Something like the recently announced Focusrite Saffire Pro 40 would give you all of this in a single unit, and although I've not used it myself I hear good reports, and it looks to be very decent value. Alternatively, you could choose an interface with eight line inputs and use external mic preamps — or, of course, a mixer. There are so many interfaces you could use with this, at so many different prices, that I can't really list them here, but most recent 24-bit interfaces should be up to the job.

If you want to use an analogue mixer to feed the line inputs of your interface, I'd suggest something like the Mackie 1642 VLZ3, taking a feed from its direct out sockets. It will probably have more functions than you need, but I doubt you'll find eight preamps of this quality in anything cheaper — the VLZ3 series preamps are neutral sounding and offer plenty of headroom. Mixers, of course, can be useful if you want to add some processing while recording — meaning that if you like to tweak EQ and compress to get the right sound before recording, so that you only need to do smaller tweaks in the computer, a mixer would still be a good option. Personally, unless you're used to working in that way, I'd opt to do that in the computer, but mixers can also double up as monitor control systems, without running the risk of latency in your computer, which is useful if you're doing a lot of recording. Alternatively, choosing a mixer with Firewire or USB2 connectivity would mean there's no need for a separate audio interface. Something like the Alesis Multimix Firewire 16 would do the job, offering you all the preamps and I/O that you need to record your eight tracks simultaneously. Whichever option you choose, you just need to select the appropriate audio interface driver in your DAW and assign the physical input channels of your interface to separate software input channels in your DAW. In this way you can record several tracks simultaneously, so that you can do whatever you want with them in your DAW software.

Speaking of your DAW, I must admit that I'm not that familiar with Soundtrack Pro. I believe it offers sufficient input tracks and I know that you can do plenty of processing in there, but I suspect it will lack some of the useful features commonly found in music-oriented DAWs such as Logic, Cubase and Pro Tools. You could certainly do worse than purchasing Logic, as it comes with a great bundle of plug-ins as standard and the current price is very competitive. If that's too pricey, there's always Logic Express, from which you can upgrade to Logic Pro when you can afford it.


Published December 2008

The New Korg Kronos: Video Manual Part 5 - Audio Recording & Sampling

Q. How can I simulate a 12-string guitar sound in the studio?

By Various

I have someone coming into my studio to do some recording and I'm looking for a way to achieve a 12‑string guitar sound for him from a six‑string acoustic. I've looked for freeware plug‑ins but haven't had much luck.

Stephen Scott

Via email

 
Generating the distinctive sound of a 12‑string guitar when you only have a six‑string might seem straightforward with the array of technology we have access to these days, but it's probably best done by rather more mechanical and low‑tech means.

SOS Editor In Chief Paul White replies: There's no ideal way to do this, as even the best octave doublers double all the string pitches, whereas on a 12‑string guitar, the top two strings are tuned in unison. Should you wish to experiment with octave doublers, be aware that the vast majority of them are based on current pitch‑shift technology, which tends to sound a little warbly and thin on all but the most expensive processors. The reason for this is that pitch shifters work by breaking the sound into very short segments, then shortening them or looping them depending on whether the pitch shift is to be up or down. Where the pitch is to be increased by an octave, each audio segment is sped up to twice its normal speed, which makes it half its original length, so then it has to be used twice to fill the original time. This works OK on smooth sounds, but can give a doubling effect on the attack of notes. You can also hear an unnatural shimmering effect where the segments are rejoined.

More sophisticated pitch‑shifters try to minimise the damage to note attacks, but the most effective of these tend to be in software plug‑ins or designed to work on single‑note guitar lines. For all that, I've never managed to create a pitch‑doubling effect for guitar that sounds much like a 12‑string (other than with a Line 6 Variax or Roland VG system, where the strings are processed separately), and you'll often get a more realistic sound by just using the pitch‑shifter to add a slightly detuned version (detuned by somewhere between five and 10 cents) to the original. This produces a natural chorus‑like effect but doesn't replicate the effect of the octave strings — it sounds more like a 12‑string guitar where all the pairs are tuned to unison.

I think your best bet is to double the guitar part, with one pass played straight and one with an acoustic guitar restrung for Nashville tuning, where the bottom three strings (or four, if you prefer) are strung with thinner strings and tuned an octave above normal. If this isn't possible, try doubling the part with the second pass played using a capo, so that the chords sound in a higher position.

Published January 2009

The New Korg Kronos: Video Manual Part 4 - Sequencer and Effects

Q. Which small mixers have high impedance inputs?

By Various

I recently read Mike Senior's review of the Alesis Multimix 16, and he listed as one of the cons the lack of a high‑impedance instrument input. I agree with him, but have been searching for ages for a small mixer with this kind of input and can't find one anywhere. Is there one? The mixer isn't actually for me, but for a musician friend who still records to eight‑track cassette via an old PA and various pedals and outboard. He plays bass pedals and has an ancient drum machines, plus he sings and plays guitar and banjo. The outboard and the PA mixer are noisy, and as I transfer his tapes and mix them I spend far too much time on noise reduction and not enough on mixing and production. He is not technically savvy, and getting him to use several of the eight‑track inputs is not a option, so I am looking for a mixer he can plug his instruments into and get a clean relatively noise‑free signal from, to feed the eight‑track.

SOS Forum post

 
Not as many small mixers as you would imagine come with high-impedance instrument inputs. Soundcraft's Compact 10 is one of them.

SOS contributor Mike Senior replies: High‑impedance inputs aren't tremendously common on traditional small‑format mixers, and it was only really because the Alesis Multimix 16 also offered USB audio interfacing for recording purposes that I felt that this omission was a significant missed opportunity. Unlike mixers, a lot of audio interfaces do have instrument inputs built in, and these products inevitably compete to a certain extent with the Multimix for the punter's pound. However, if you root around a bit you can still find a few products that might fit the bill.

For example, Mackie's latest 402 and 802 VLZ3 mini‑mixers both have two line inputs that are switchable for high‑impedance operation, and also have the company's renowned XDR2 mic preamps, so those would probably be well worth looking at. I can see their relative operational simplicity being a big part of the appeal here too. The same company's newly announced U420 four‑channel mixer/interface also has a high‑impedance input, but I don't think you really need that unit's Firewire interfacing, so it wouldn't represent good value for you in this case.

Another option might be the Soundcraft Compact 4 or Compact 10, with one and two high‑impedance inputs respectively, and these should also be pretty easy to drive. Edirol's M16DX and M10DX digital mixers also have high‑impedance inputs, but I'd probably steer clear of those in this situation, simply because the potential benefits of their built‑in digital processing will almost certainly be lost on someone dealing primarily with analogue gear and not particularly technically minded. Other than those models, there doesn't appear to be much else out there with an instrument input built in, but I think we're going to see that changing, to be honest, as the line between 'mixer' and 'audio interface' becomes ever more blurred.

Published February 2009